Dr. G. J. Hinde — Organic Origin of Chert. 445 



frequently suspected by others, but no satisfactory proof has been 

 heretofore brought forward to decide the question. I think, however, 

 that the evidence which I have now collected from so many 

 quarters will be found sufficiently conclusive to set at rest all 

 objections to their organic origin. 



I am not in a position to discuss how far the British Cherts cor- 

 respond with the phthanites of the Carboniferous strata of Belgium, 

 but judging from the careful description given of the latter by 

 M. Eenard, the resemblance is very close, and in my opinion the 

 phthanites will be found to have the same origin as our Cherts. 

 I have some grounds for this belief from the fact that in sections 

 of a hand specimen of phthanite from the Carboniferous rocks at 

 Namur, which was kindly sent to me by Dr. C. Barrois, there are 

 abundant remains of sponge-spicules, and in transverse sections 

 they correspond closely with those figured by M. Renard in illus- 

 tration of his paper. Most of these spicules, however, are now 

 so fragmentary and altered, that they would not be recognized as 

 such by any one unfamiliar with the changes produced in them by 

 fossilization. 



I wish in conclusion shortly to notice some of the arguments 

 brought forward by Messrs. Hull and Hardman and by M. Renard 

 in favour of what may be called the chemical theory of Chert, 

 which is that the silica of which it is composed is a direct deposit 

 from the stores of this mineral held in solution in the sea-watei', 

 without the intervention of organic life. According to the explana- 

 tion given by M. Renard,^ at certain intervals the waters of the 

 Carboniferous sea, holding in solution carbonic acid, a solvent of 

 calcite, attacked the calcareous material, the silicic acid being infil- 

 trated into it, and taking its place in proportion as it was decom- 

 posed. Prof. Hull distinctly states that the Chert is essentially 

 a pseudomorphic rock consisting of gelatinous silica replacing lime- 

 stone of organic origin. 



I am unable to recognize the evidence for the statement that the 

 silica of the Carboniferous Chert is a pseudomorph after limestone. 

 In the beds of pure Chert, that is, when there is scarcely any 

 admixture of carbonate of lime, hardly any traces of organisms other 

 than sponge-spicules can be distinguished. According to the trans- 

 mutation theory these Chert-beds were originally organic limestones, 

 and should now show in silica the forms of their original structures; 

 but, unfortunately for the theory, there are scarcely any present in 

 them. In the Cherty limestones, calcareous organic structures are 

 plentiful, but curiously enough, even here, there has been little 

 replacement. Silica has indeed infilled the vacant spaces in the 

 cells of the Polyzoa and the stems of the Crinoids, but it has not, as 

 a rule, replaced the calcareous structures themselves. This is clearly 

 shown when the Cherty limestones are exposed to atmospheric in- 

 fluences by which the calcareous structures in them are dissolved 

 away and removed, leaving perfect moulds of their forms in the 

 siliceous matrix, which is now in the form of rottenstone. If they 

 1 Bullet, de I'Acad. Eoy. de Belgique, 2me s. t. 46 (1878), p. 497. 



